Secrets and several other past memories that you prefer not to talk about with anyone else. This can range from one-night stands to betraying a best friend to as much as killing someone. However, "skeletal" moments don't just have to do with only dark pasts. It can also be about embarrassing moments you've had that you simply feel too awful to speak of.
Someone with barely any or no skeletons in the closet can make it far in a large corporation.
Being married has taught me that if I don't clean up the skeletons in the closet sooner than I should can have abrupt consequences.